With the recent reduction in size of semiconductor elements such as transistors, trench isolation has been adopted as a technique for isolating transistors in semiconductor integrated circuits. Specifically, this trench isolation is a technique for isolating adjacent active regions by forming a trench (groove) in a surface region of a semiconductor substrate to define the active regions and then filling the trench with an insulator film.
In particular, in a semiconductor device using a silicon on insulator (SOI) substrate with a structure in which a semiconductor layer for forming active regions therein is provided on an insulating layer, if a trench is formed to reach the insulating layer, isolation can be made as intended. For example, in Japanese Laid-Open Publication No. 61-059852, a trench is formed in a semiconductor layer to reach an insulating layer, using an SOI substrate formed by a bonding technique, thermal oxide films (side insulating films) are formed on the sides of the trench, and then a gap between the side insulating films formed on the sides of the trench is filled with polycrystalline silicon, thereby forming a trench isolation region.
However, if a high voltage is applied between active regions at both sides of the isolation region in the known semiconductor device disclosed in the above publication, the high voltage is intensively applied to the thermal oxide films (thermal oxide films) in the isolation region, so that the device exhibits low breakdown strength. On the other hand, if the thermal oxide films are made thick enough, the dielectric breakdown strength increases, but crystal defects occur in semiconductor crystal in the active regions. These crystal defects occur because when the thick thermal oxide films are formed in a thermal oxidation process performed for a long time, the volume of the thermal oxide films increases so that the active regions undergo a large stress. The occurrence of the crystal defects might deteriorate the current-driving capability of the transistors.
In a semiconductor device having an isolation region formed by filling the entire trench with an insulator such as a CVD oxide film, a large stress resulting from the difference in thermal expansion coefficient between the insulator and a semiconductor might also be applied to active regions.